Capillary electrochromatography (CEC) is a hybrid method of capillary electrophoresis (CE) and high performance liquid chromatography (HPLC). Though CEC was first demonstrated more than two decades ago, the advent of sophisticated CE instrumentation, expanded use and understanding of CE, and the continuing quest for more efficient separation methods has recently intensified interest in CEC.
CEC involves the application of an electric field between the ends of a 50-110 .mu.m capillary containing a stationary phase. "Open tubular" CEC describes a technique where the stationary phase is bonded to the capillary wall, while "packed" CEC describes a method involving capillaries filled either with a polymer gel stationary phase or a small particle (about 1-10 .mu.m) silica-based stationary phase.
In all CEC techniques, a liquid phase is transported through the capillary by electroosmosis or a combination of electroosmosis and pressure, and solutes are separated based on their partitioning between the stationary and mobile phases and on their charge to frictional drag. As shown in FIG. 1, the electroosmotic flow originates from the electrical double layer at the surface of the stationary phase as well as the capillary wall and generates a plug-like flow profile which is independent of the geometry and size of the channels between the particles. This phenomenon can provide very high efficiencies, limited primarily by the solute diffusion coefficient.
In contrast to capillary liquid chromatography, CEC can utilize long capillaries of small, very efficient particles since there is no column back pressure. It has been shown in Dittmann et al., LC-GC, 13: 800 (1995) that CEC has the potential to provide column plate numbers 5 to 10 times greater than HPLC columns. The high efficiencies attainable make CEC a very attractive technique for chiral separations since it is theoretically possible to obtain baseline resolution for solutes with very small enantioselectivities.
Polysaccharide derivatives coated onto porous derivatized silica have proven to be among the most versatile and widely used chiral stationary phases in HPLC. They have been used in both normal and reversed phase mode and have shown extremely high enantioselectivity for many solutes. Unfortunately, unlike several other chiral selectors, their use as buffer additives in CE is precluded by their poor solubility in suitable electrolytes and high UV cut-off. However, these characteristics do not preclude their use as chiral stationary phases in CEC, and open tubular electrochromatography using 50 .mu.m I.D. fused silica capillaries coated with a cellulose derivative has been investigated by E. Francotte and M. Jung, Chromatographia, 42: 521-527 (1996). Resolution was found to be heavily dependent on the thickness of the coating, and the highest efficiency achieved was a disappointingly low 60,000 plates/m.